循环解析后的 reloadData

Question

我正在尝试在此循环完成时重新加载 tableView，但是每当我尝试将 self.tableView.reloadData() 放在循环之外时，循环返回 0。我想这是因为我得到后台的数据。循环完成后我该怎么做才能重新加载。

func loadData() {
    var query = PFQuery(className: "Items")
    query.includeKey("user")
    query.getObjectInBackgroundWithId(itemId) {
        (itemObject: PFObject!, error: NSError!) -> Void in
        if (error == nil) {


            var userObject = itemObject.objectForKey("user") as PFObject


            let userImageFile = userObject.objectForKey("file") as PFFile
            userImageFile.getDataInBackgroundWithBlock {
                (imageData: NSData!, error: NSError!) -> Void in
                if error == nil {
                    let image = UIImage(data:imageData)






                    self.detailDic?.setObject(itemObject.objectForKey("title"), forKey: "title")
                    self.detailDic?.setObject(itemObject.objectForKey("description"), forKey: "desc")
                    self.detailDic?.setObject(itemObject.objectForKey("location"), forKey: "point")
                    self.detailDic?.setObject(userObject.objectForKey("name"), forKey: "name")
                    self.detailDic?.setObject(userObject.objectForKey("gender"), forKey: "gender")
                   self.detailDic?.setObject(image, forKey: "image")

                    var relation = itemObject.relationForKey("file") as PFRelation
                    var imageQuery = relation.query() as PFQuery
                    imageQuery.findObjectsInBackgroundWithBlock { (imageObj: [AnyObject]!, error1: NSError!) -> Void in
                        if error1 == nil {

                            var imageDic:NSMutableArray = NSMutableArray()

                            let group = dispatch_group_create()

                            for obj in imageObj {
                                var imageObject = obj as PFObject
                                var thumbnail = imageObject.objectForKey("file") as PFFile
                                // Important: enter the group *before* starting the background task.
                                dispatch_group_enter(group)
                                thumbnail.getDataInBackgroundWithBlock {
                                    (imageData: NSData!, error: NSError!) -> Void in
                                    if error == nil {
                                        let theImage = UIImage(data:imageData)
                                        theImage.CGImage
                                        self.imageArray!.addObject(theImage)
                                        dispatch_group_leave(group)
                                    }
                                }
                            }

                            dispatch_group_notify(group, dispatch_get_main_queue()) {
                                self.tableView?.reloadData()
                            }







                        }


                    }



                }

            }




        }


    }

}

Answer 1

使用调度组。调度组基本上是一个计数器。您可以原子地递增和递减它，并且您可以要求在它为零时得到通知。

每次添加后台任务时，“进入”组（其计数器递增）。当后台任务完成时，“退出”组（递减计数器）。当计数器为零时使用dispatch_group_notify 运行一个块（意味着所有后台任务都已完成）。

let group = dispatch_group_create()

for obj in imageObj {
    var imageObject = obj as PFObject
    var thumbnail = imageObject.objectForKey("file") as PFFile
    // Important: enter the group *before* starting the background task.
    dispatch_group_enter(group)
    thumbnail.getDataInBackgroundWithBlock {
        (imageData: NSData!, error: NSError!) -> Void in
        if error == nil {
            let theImage = UIImage(data:imageData)
            self.imageArray!.addObject(theImage)
            dispatch_group_leave(group)
        }
    }
}

dispatch_group_notify(group, dispatch_get_main_queue()) {
    self.tableView?.reloadData()
}

请注意，如果imageObj 中没有元素，或者如果所有后台任务在dispatch_group_notify 运行之前完成，dispatch_group_notify 将 立即运行该块（这是您想要的)。

另外，UIImage 不一定在您创建图像数据时对其进行解码。它可以懒惰地这样做。您可能希望强制它在后台线程上解码图像数据。你可以通过询问它的CGImage来做到这一点：

        if error == nil {
            let theImage = UIImage(data:imageData)
            theImage.CGImage // force decoding immediately
            self.imageArray!.addObject(theImage)
            dispatch_group_leave(group)
        }

Answer 2

你也可以这样做：

for (index,obj) in enumerate(imageObj) {

    .......

    // reload after last loop 
    if(index == imageObj.count) {
        self.tableView?.reloadData()
    }

}